U.S. patent application number 10/282461 was filed with the patent office on 2003-05-15 for easy to disperse, high durability tio2 pigment and method of making same.
Invention is credited to Bettler, Charles Robert, Deibold, Michael Patrick.
Application Number | 20030089278 10/282461 |
Document ID | / |
Family ID | 26961464 |
Filed Date | 2003-05-15 |
United States Patent
Application |
20030089278 |
Kind Code |
A1 |
Bettler, Charles Robert ; et
al. |
May 15, 2003 |
EASY TO DISPERSE, HIGH DURABILITY TIO2 PIGMENT AND METHOD OF MAKING
SAME
Abstract
The present invention relates to a TiO.sub.2 pigment coated
sequentially in a wet treatment process with hydrous silica and
hydrous alumina both in the presence of citric acid wherein the
resulting pigment is coated with from 1 to 4% amorphous alumina
based on the weight of the untreated TiO.sub.2; from 3 to 6% silica
glass based on the weight of the untreated TiO.sub.2.
Inventors: |
Bettler, Charles Robert;
(Newark, DE) ; Deibold, Michael Patrick;
(Wilmington, DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY
LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1128
4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
26961464 |
Appl. No.: |
10/282461 |
Filed: |
October 29, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60319004 |
Nov 1, 2001 |
|
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|
Current U.S.
Class: |
106/442 |
Current CPC
Class: |
C01G 23/047 20130101;
C09D 5/035 20130101; C09D 7/62 20180101; C01P 2006/90 20130101;
C08K 3/22 20130101; C09C 1/3669 20130101; Y10T 428/2993 20150115;
C08K 9/02 20130101; C01P 2006/60 20130101; C09C 1/3661 20130101;
C01P 2004/82 20130101; C09D 167/08 20130101; Y10T 428/2991
20150115; C09C 1/3653 20130101; C09C 1/3684 20130101; C08K
2003/2241 20130101 |
Class at
Publication: |
106/442 |
International
Class: |
C09C 001/36; C04B
002/00 |
Claims
What is claimed is:
1. A method for making an easy-dispersing, high durability titanium
dioxide pigment comprising the steps in sequence: A. (1) heating a
slurry of titanium dioxide particles to a temperature of from 85 to
100.degree. C., (2) adding citric acid as a solution in water to
the slurry to form a mixture, (3) adjusting the pH of the mixture
to 10 or more, (4) adding sufficient sodium silicate as a water
solution to the mixture to deposit silica on the surface of the
particles of from 1 to 3% based on the weight of the titanium
dioxide particles in the slurry, (5) neutralizing the slurry by
addition of a mineral acid over the course of one hour, thereby
forming a slurry of silica coated titanium dioxide particles; and
B. (1) adjusting the temperature of the slurry of silica coated
titanium dioxide particles to a temperature of from 55 to
90.degree. C., (2) adding sufficient sodium aluminate as a water
solution to the slurry in step B(1) and adjusting the pH of the
mixture formed to from 5 to 9 by addition of a strong mineral acid
to deposit aluminia as Al.sub.2O.sub.3 of from 1 to 4% by weight
based on the weight of titanium dioxide particles present in the
slurry of step A(1) on the surface of the silica coated particles,
and digesting the resulting mixture for from 15 to 30 minutes.
2. The method of claim 1 where the method is used as a batch or a
continuous process.
3. The method of claim 1 wherein step A (1) the slurry is heated to
a temperature of from 90 to 95.degree. C.
4. The method of claim 1 wherein the citric acid is added to the
slurry to a concentration based on the weight of the titanium
dioxide particles of from 0.1 to 2%.
5. The method of claim 4 wherein the citric acid is added to a
concentration of 0.3 to 0.5%.
6. The method of claim 1 wherein the strong mineral acid is
selected from the group consisting of nitric, hydrochloric and
sulfuric acids.
7. The method of claim 1 wherein step B(2) the addition of sodium
aluminate solution and the pH adjustment by addition of mineral
acid is done simultaneously.
8. An improved method of making a titanium dioxide particles
wherein the particles in a water slurry are wet treated with water
solution of sodium silicate and sodium aluminate to form titanium
dioxide having a first coating of silica followed by a second
coating of aluminia wherein the improvement comprises adding citric
acid to the titanium dioxide slurry before the addition of sodium
silicate solution.
Description
BACKGROUND OF THE INVENTION
[0001] Addition, during TiO.sub.2 surface treatment, of certain
chemicals under specific conditions significantly enhance a number
of pigment performance properties in paints. These properties
include durability and ease-of dispersion. The chemicals added
during surface treatment that improve pigment end-use performance
include metal oxides, particularly aluminum oxides and silicon
dioxide. The exact nature of these oxides may in turn be influenced
by the conditions under which they are formed and the presence of
other reagents during the precipitation process. The present
invention provides an easy dispersing, high durability TiO.sub.2
pigment and method of making that pigment. Such super-durable
TiO.sub.2 pigment with improved gloss retention (durability) and
dispersibility (easy dispersing) is useful in a variety of exterior
applications, including but not limited to powder, coil,
automotive.
[0002] Some multi-purpose TiO.sub.2 pigment grades that are
commercially available are treated with a coating of amorphous
alumina. In these products there is no silica present as a surface
treatment. Although these products have durability that is
sufficient for architectural exterior coatings, they are not
super-durable. In these products materials such as zirconia and tin
are used to stabilize the amorphous alumina. The use of zirconia
and tin makes these grades more expensive to manufacture and to
use. The present invention uses citric acid to stabilize the
amorphous alumina. The combination of silica and citric acid
stabilized amorphous alumina on a pigment seems to be a new
approach that provides performance and cost efficiencies.
[0003] U.S. Pat. No. 3,825,438 teaches a process to make a coated
titanium dioxide pigment with one or more hydrous oxides in the
presence of a polyhydric alcohol and/or a carboxylic acid. In this
process, the alcohol or acid may be added at any time in the
process prior to the completion of the deposition of the coating.
The teachings in U.S. Pat. No. 3,825,438 solve the problem of
pigment agglomeration on storage. A broad series of organic
additives are taught in this patent to accomplish this desired
result. A critical teaching is that the point at which the alcohol
and/or acid are added in the process has no influence on the
results achieved in the pigment product.
[0004] U.S. Pat. No. 5,340,393 teaches a method of making a
non-agglomerated water insoluble inorganic particle having a dense,
amorphous silica coating. Citric acid is identified as a possible
optional charge stabilization dispersing acid that may be used in
the process to prevent agglomeration of particles in the process.
This patent offers no teaching as to the deposit of amorphous
alumina or special properties resulting from the use of citric acid
over other dispersants taught.
[0005] U.S. Pat. No. 5,730,796 teaches a process to make a durable
pigment having a coating of cerium oxide, a coating of dense,
amorphous silica and preferably an outer coating of crystalline
(hydrous) alumina. It makes no teaching or suggestion with respect
to the use of citric acid in deposition of silica or aluminia to
enhance pigment properties
[0006] U.S. Pat. No. 5,700,318 teaches an alumina coated inorganic
pigment having a first coating of boehmite alumina, a second
coating of amorphous alumina and a third coating of boehmite
alumina. Adjustment of precipitation pH is taught as a means to
produce the crystalline and amorphous alumina coatings. No teaching
or suggestion is made with respect to the use of citric acid in
this process.
[0007] U.S. Pat. No. 3,506,466 teaches the use of a salt of a water
soluble alcohol amine and an easily water soluble oxycarboxylic
acid to enhance dispersibility. The salt is added after wet
treatment and before grinding the pigment.
[0008] U.S. Pat. No. 5,824,146 teaches a titanium dioxide pigment
with amorphous alumina product in vapor phase oxidation of titanium
tetrachloride in the presence of aluminum trichloride. The pigment
is slurried using a citric acid dispersant.
[0009] U.S. Pat. No. 5,824,145 teaches mixtures of titanium dioxide
pigment slurry and slurry of amorphous alumina hydroxide formed
from mixing the two component slurries and using citric acid as a
dispersing agent foe the mixed slurries. There is no wet treatment
process disclosed in this patent.
[0010] U.S. Pat. Nos. 2,885,366 and 3,437,502 provide the basic
teachings for making a durable titanium dioxide pigment having an
amorphous silica coating followed by a coating of mixed crystalline
alumina. Neither of these patents teach the use of citric acid
during the wet treatment process used to deposit these treatments
on the pigment surface.
BRIEF SUMMARY OF THE INVENTION
[0011] The present invention relates to a method for making an
easy-dispersing, high durability titanium dioxide pigment
comprising the steps in sequence:
[0012] A. (1) heating a slurry of titanium dioxide particles to a
temperature of from 85 to 100.degree. C.,
[0013] (2) adding citric acid as a solution in water to the slurry
to form a mixture,
[0014] (3) adjusting the pH of the mixture to 10 or more,
[0015] (4) adding sufficient sodium silicate as a water solution to
the mixture to deposit silica on the surface of the particles of
from 1 to 3% based on the weight of the titanium dioxide particles
in the slurry,
[0016] (5) neutralizing the slurry by addition of a mineral acid
over the course of one hour, thereby forming a slurry of silica
coated titanium dioxide particles; and
[0017] B. (1) adjusting the temperature of the slurry of silica
coated titanium dioxide particles to a temperature of from 55 to
90.degree. C.,
[0018] (2) adding sufficient sodium aluminate as a water solution
to the slurry in step B(1) and adjusting the pH of the mixture
formed to from 5 to 9 by addition of a strong mineral acid to
deposit aluminia as Al.sub.2O.sub.3 of from 1 to 4% by weight based
on the weight of titanium dioxide particles present in the slurry
of step A(1) on the surface of the silica coated particles, and
digesting the resulting mixture for from 15 to 30 minutes.
[0019] The method of the present invention may be used as a batch
or a continuous process. In practicing the method it is preferred
that in step A (1) the slurry is heated to a temperature of from 90
to 95.degree. C. It is also preferred that citric acid be added to
the slurry to a concentration based on the weight of the titanium
dioxide particles of from 0.1 to 2%, and even more preferred that
citric acid is added to a concentration of 0.3 to 0.5%.
[0020] The strong mineral acid used in the present invention is
selected from the group consisting of nitric, hydrochloric and
sulfuric acids. The most preferred acid to use when treating
pigment made by the chloride process is hydrochloric acid, and the
most preferred acid to use when treating pigment made by the
sulfate process is sulfuric acid.
[0021] In the method of the present invention it is preferred in
step B(2) that the addition of sodium aluminate solution and the pH
adjustment by addition of mineral acid is done simultaneously.
[0022] The present process offers an improved method of making a
titanium dioxide particles wherein the particles in a water slurry
are wet treated with water solution of sodium silicate and sodium
aluminate to form titanium dioxide having a first coating of silica
followed by a second coating of aluminia wherein the improvement
comprises adding citric acid to the titanium dioxide slurry before
the addition of sodium silicate solution.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention provides a product and a process for
making that product in which TiO.sub.2 is coated sequentially in a
wet treatment process with amorphous silica and amorphous alumina
following procedures similar to those currently employed except
that citric acid is added during the deposition process.
[0024] In a typical application of this invention, slurry of
TiO.sub.2 is heated to a predetermined temperature and citric acid
is added. The slurry is then pH adjusted to a target and sodium
silicate is added. The pH is slowly decreased to neutral by
addition of acid, after which the slurry is allowed to digest for a
period of time, at the end of which the pigment particles are well
coated with a contiguous layer of silica. The temperature of the
slurry is then changed, if desired, and sodium aluminate and acid
are then added together in such a way that the final pH after
reagent addition is controlled to a predetermined value. The slurry
is allowed to digest for a certain amount of time, then processed
into a finished material by well-established methods.
[0025] The following outlines the operation limits and ranges of
the present process. Step 1 is the silica deposition. Level of
citric acid is typically 0.5% citric acid based on weight of
untreated TiO.sub.2.
[0026] The concentration of TiO.sub.2 in the slurry ranges from 200
to 450 grams per liter, although lower levels are also possible.
The temperature is optimally from 85 to 100.degree. C., although
lower temperatures might also be effective.
[0027] The pre-silica deposition pH is typically above 10.0
although this may depend on the equipment used (lower pH is
possible for continuous wet treatment). The optimal silica
deposition weight is between 3 and 6% as SiO.sub.2 based on weight
of untreated TiO.sub.2. However, improvements due to the invention
are likely to be seen at any silica level.
[0028] Any strong mineral acid, including HCl, HNO.sub.3 and
H.sub.2SO.sub.4 may be used for neutralization. The optimal acid
addition time for batch process is 30 minutes per 1% SiO.sub.2
added. Longer times lead to better product at the expense of rate.
Digestion time is typically 15 minutes per 1% silica. Shorter times
lead to slightly worse product made at higher rate.
[0029] Step 2 alumina deposition: The initial temperature is
optimally 55 to 90.degree. C., although lower temperatures might
also be effective (or even more effective. However, this requires
chilling of the slurry at the expense of energy and time).
Aluminate amount is Optimal between 1 and 4% as Al.sub.2O.sub.3
based on weight of untreated TiO.sub.2. Lower levels will begin to
degrade pigment performance properties (ease of dispersion;
dispersion stability; gloss), as will higher levels (tint strength,
hiding power, gloss). Any strong mineral acid, including HCl,
HNO.sub.3, and H.sub.2SO.sub.4 may be used.
[0030] Post addition pH: Optimal is between 5 and 9. Higher values
might lead to undesired alumina phase; lower values to incomplete
deposition. Digestion time is typically 15 to 30 minutes. Short
times might lead to incomplete mixing/reacting of ingredients; long
times needlessly decrease rate through the process.
[0031] Pigment compositions of the present invention include from 1
to 4% amorphous alumina based on the weight of the untreated
TiO.sub.2; from 3 to 6% silica based on the weight of the untreated
TiO.sub.2 where the silica is in the form of a silica glass.
[0032] Pigments of the present invention provide durability equal
to that of DuPont's R-960 grade, but with dispersion qualities
superior to an architectural pigment grade combined with gloss
retention much improved over that of such durable grades as R-960.
This combination of properties is reflected by the term
super-durable as a description of pigment type.
Test Methods
[0033] Acid Solubility Test:
[0034] 10 mL 66% sulfuric acid is added to a test tube containing a
magnetic stirring bar, and the tube is placed in an aluminum
heating block and heated to 175.degree.. A 0.2000 g pigment sample
is added to the tube, and digested for 1 hour with stirring. At the
conclusion of the digestion period, the tube containing the pigment
sample is cooled by pouring the acid mixture into a beaker of ice,
and the residue in the tube and beaker is washed with distilled
water. The pigment residue in the tube and beaker is collected in a
100 mL volumetric flask, the volume made up to 100 mL by addition
of distilled water, and the contents mixed thoroughly. The contents
of the volumetric flask are then filtered, and 10 mL of the
resulting filtrate are added to a 25 mL volumetric flask, to which
2 mL 20% hydrogen peroxide, and sufficient 10% sulfuric acid to
make 25 mL, are added. The resulting solution is allowed to stand
one hour. Absorbance of the solution is then read at 400 nm using a
10 mm cell path. Soluble TiO.sub.2 is determined by reference to a
previously prepared spectrophotometric curve obtained by
measurement of samples containing known quantities of dissolved
TiO.sub.2.
[0035] Dispersibility Testing:
[0036] The dispersibility of the TiO.sub.2 pigment made according
to Example 2 was determined and compared to commercially available
TiO.sub.2 pigments. Generally speaking, this determination was made
by dispersing the pigments in an alkyd vehicle under two different
controlled conditions that spanned a wide range of dispersion
energies. The resulting dispersions were then let down with
additional vehicle and tested for cleanness (number of undispersed
particles) using a Hegman gauge. It is desirable that this number
is minimal, and that low value is achieved under the mildest grind
conditions. A more exact description of this procedure is provided
in the text of Example 2.
EXAMPLES
Example 1
[0037] A rapidly stirred slurry of 2000 grams of TiO.sub.2
particles in 5.5 liters of water was heated to 90.degree. C., after
which the pH was adjusted to 10.0 using a solution of sodium
hydroxide in water. Citric acid (10 grams) was added and the pH of
the slurry adjusted to 10.5. A solution of sodium silicate (125 ml,
concentration equivalent to 400 g SiO.sub.2 per liter) was added
and the pH then lowered to 7.0 over the course of two hours by
carefully controlled addition of 5 molar hydrochloric acid. The
neutralized slurry was aged with stirring for 45 minutes while pH
and temperature were maintained at 7.0 and 90.degree. C.,
respectively. After aging, 175 ml of sodium aluminate, of
concentration equivalent to 420 g Al.sub.2O.sub.3 per liter, was
added. During this addition hydrochloric acid was also added such
that the pH remained between 6.0 and 8.0 throughout. Slurry pH was
adjusted to 7.0 and the slurry aged with stirring for 20 minutes.
The pigment was filtered, washed, and dried overnight at
105.degree. C. The dry pigment was screened through a 12 mesh
sieve, then ground in an 8 inch micronizer.
Comparative Example 1A
[0038] Like example 1, except tartaric acid was used in place of
citric acid.
Comparative Example 1B
[0039] Like example 1, except ethylene glycol was used in place of
citric acid.
Comparative Example 1C
[0040] Like example 1, except tricarbolylic acid was used in place
of citric acid.
Comparative Example 1D
[0041] Like example 1, except citric acid was omitted.
SUMMARY OF RESULTS
[0042] Pigment was subjected to the acid solubility test, which
measures pigment durability, with the following results shown in
Table 1.
1TABLE 1 Comparison of the Effect of Carboxylic Acid Selection on
Acid Solubility of the Pigment Example Acid Solubility 1 .sup. 10.2
1A 17.5 1B 19.0 1C 18.2 1D 16.1
[0043] Data shown in Table 1 show that the use of citric acid in
the pigment wet treatment resulted in a pigment having a
substantially enhanced durability as measured by the acid
solubility test--clearly a surprising result.
Example 2
[0044] This Example illustrates the improved dispersion quality of
the pigment of the present invention. All solution used in the
process were solutions in water.
[0045] 2800 pounds raw TiO.sub.2 (chloride process pigment
collected before any wet treatment processing) was diluted with
water to give a final TiO.sub.2 concentration of 240 grams per
liter and heated to 95.degree. C. via steam injection. A 50 wt %
solution of citric acid (17 pounds; 0.3% citric acid based on
weight of TiO.sub.2) was added. The pH of the resulting mixture was
adjusted to pH greater than 10.0 using concentrated sodium
hydroxide solution (50% by weight in water).
[0046] A solution of sodium silicate was added (27.2 gallons; 3.0%
SiO.sub.2 based on weight of TiO.sub.2) and the pH of the resulting
slurry was lowered to 7.0 over the course of 1 hour using a
solution of 20% hydrochloric acid while maintaining the temperature
at ca. 95.degree. C. using steam. Once the addition of the acid was
complete, the steam was turned off and the slurry was allowed to
digest for one hour. Then, sodium aluminate (28.3 gallons; 2.6%
Al.sub.2O.sub.3 based on weight of TiO.sub.2) and hydrochloric acid
were added together over a 20 minute period in a way that kept the
pH of the slurry close to neutral. The slurry was allowed to digest
for 30 minutes at the end of this addition, then the pH was
adjusted to between 5.0 and 5.5. The resulting slurry was filtered
and washed sufficiently to give a final pigment resistance of
greater than 8 k-ohm. The filtered pigment was dried and ground
using a fluid energy mill (micronizer).
[0047] Trimethylolpropane (0.18% C based on weight of TiO.sub.2)
was added to the pigment prior to grinding.
[0048] Pigment Dispersion was Tested as Follows:
[0049] A mixture of 278 grams Beckosol 10-060 long oil soya alkyd
resin (65% solids) and 362 grams TiO.sub.2 pigment were added to a
1000-mL polyethylene tri-pour beaker. Using a 6" spatula, the
pigment was blended with the liquid resin until it was fully wet
in. The tri-pour beaker was then placed in a stainless steel,
water-jacketed pot that was positioned within a laboratory
Hockmeyer Disperser. A 3" Cowles blade was lowered to within 1/4"
of the bottom of the beaker and the pigment/resin mixture was
ground at 650 RPM for one minute. After grinding, 106 g of this
grind base and 119 g of Beckosol 10-060 resin were added to a metal
paint can and shaken on a paint shaker for 10 minutes. Degree of
dispersion was then measured using a Hegman gauge, where the paint
was drawn down and the number of undispersed particles between
Hegman value 7 and 4 for counted. For poorly dispersed samples,
counting was discontinued once this number reached 40 and was
simply reported as 40+. Each sample was measured multiple times and
the average count reported. The results of this measurement
indicated the ease with which the pigment dispersed into the resin
using low dispersion energy. Lower counts indicate a greater ease
of dispersion.
[0050] A second dispersion measurement was made for all pigments in
a manner similar to that described above except the pigment/resin
mixture was ground for three minutes at 650 RPM followed by five
minutes at 1750 RPM followed by five minutes at 3000 RPM. The
results of this second procedure indicated the ultimate performance
that could be expected after a very energy intensive grind.
[0051] Results for pigment made according to Example 2 and several
commercially available pigments were compared for their ease of
dispersion. Results of these tests are given in Table 2. In this
Table, commercial grades A through C contain surface treatments of
silica and alumina; Commercial Grades D and E contain surface
treatments of primarily alumina and zirconia. Commercial Grades B
through E contain on their surfaces an organic dispersion aid (c.
f. the Trimethylolpropane added to pigment Example 2). The data in
this Table demonstrates that with enough energy most pigments will
ultimately disperse well into the resin (e.g., have count numbers
below 15). However, only the pigment made according to our
invention disperses well at low grind energies. That is, pigment
made according to the present invention displays an
ease-of-dispersion that is not present in the commercial grades
described above.
2TABLE 2 Ease of Dispersion Comparison: Pigment of the Invention
and Commercially Available Universal or Durable Grade Products
TiO.sub.2 Pigment Low Energy, Count High Energy, Count Example 2
3.2 2.7 Commercial A 40+ 5.7 Commercial B 40+ 5.0 Commercial C 40+
3.0 Commercial D 40+ 8.7 Commercial E 40+ 25.2
* * * * *